This invention relates to an electrical apparatus such as a high-power electrical transformer.
FIG. 1 is a sectional view illustrating a conventional three-phase transformer, as an example of an electrical apparatus used in a power plant or the like, in which reference number 1 indicates a transformer main body which comprises an iron core 101 and a winding 102. 2 indicates a tank which contains the transformer main body 1 and which is filled with an electrically insulating oil.
FIG. 2 is a front view of the upper portion of the tank 2 as viewed from the right in FIG. 1. 201 and 202 indicate high-voltage bushing mounting seats and low-voltage bushing mounting seats formed on the tank 2, respectively, and 3 indicates high-voltage bushings mounted on the high-voltage bushing mounting seats 201 and connected to a high-voltage side of the winding 102. 203 indicates bus conductor outer sheath mounting flanges formed on the low-voltage mounting seats 202, 204 indicates low-voltage bushing mounting flanges formed on the bus conductor outer sheath mounting flanges 203, and 4 indicates low-voltage bushings mounted to low-voltage bushing mounting flanges 204 and connected to the low-voltage side of the winding 102, the detail of which is shown in FIG. 3.
In FIG. 3, 401 indicates an insulator tube, 402 indicates a circular rod-shaped central conductor inserted into the insulator tube 401, 403 and 404 indicate an upper terminal and a lower terminal disposed at the opposite ends of the central conductor 402 and 405 indicates a mounting unit disposed about the outer circumference of the central portion of the insulator tube 401 and secured thereto by cement 406. By attaching the mounting unit 405 to the low-voltage bushing mounting flange 204 by bolts 5A, the low-voltage bushing 4 is mounted so that it extends into the tank 2.
As partly shown in FIG. 2, there are three of the high-voltage and low-voltage bushings 3 and 4 and their mounting seats 201 and 202 for three phases. In FIGS. 1 and 2, 6 indicates three-phase separated bus conductors connecting the low-voltage bushings 4 and a generator (not shown), 7A indicates transformer-side three-phase separated bus conductor outer sheaths surrounding the low-voltage bushings 4 at the portion located outside of the tank 2, 8A and 8B indicate outer sheath shorting plates common to three phases and disposed at the opposite ends of the transformer-side three-phase separated bus conductor outer sheaths 7A, one of the shorting plates, 8A, being mounted on the bus conductor outer sheath mounting flanges 203. 7B indicates generator-side three-phase separated bus conductor outer sheaths surrounding the three-phase separated bus conductors 6, 8C indicates an outer sheath shorting plate common to three phases and disposed at one of the ends of the generator-side three-phase separated bus conductor outer sheaths 7B, a similar outer sheath shorting plate (not shown) being provided at the other ends. 9 indicates expansion joints made of an electrically insulating material and disposed between the transformer-side and the generator-side three-phase separated outer sheaths 7A and 7B.
The operation will now be described. An electric current which flows from the unillustrated generator through the three-phase separated bus conductors 6 and the low-voltage bushings 4 flows into the winding 102 of the transformer main body 1 and is boosted and supplied to an external circuit. Since the current from the generator is large, a massive magnetic flux is generated around the three-phase separated bus conductors 6, so that a stray loss is increased and the metal of the adjacent structural members (not shown) is overheated. Therefore, a so-called mini-flux structure is adapted, in which a three-phase closed circuit is formed by the three-phase separated bus conductor outer sheaths 7B on the generator-side, the outer sheath shorting plate 8C on one of the ends of the outer sheaths and the unillustrated outer sheath shorting plate on the other ends of the outer sheaths so that the magnetic flux crosses the closed circuit and generates a current flowing in the direction opposite to the three-phase separated bus conductors 6 thereby generating a magnetic flux which offsets the magnetic flux generated by the current flowing through the three-phase separated bus conductor 6. Similarly, the transformer-side three-phase separated bus conductor outer sheaths 7A and the shorting plates 8A and 8B form a three-phase closed circuit having a mini-flux structure. Further, the expansion joint 9 absorbs the dimensional differences between both of the three-phase separated bus conductor outer sheaths 7A and 7B. An insulating material is selected for the joint to electrically isolate the generator side and the transformer side so that they do not electrically influence each other.
Since a mini-flux structure including the phase-separated bus conductors and the outer sheath shorting plates is employed on the outside of the tank, the magnetic fluxes generated by the current flowing through the phase-separated bus conductors ara cancelled out. However, since a mini-flux structure is not adapted on the inside of the tank, a massive magnetic flux is generated by the current flowing through the bushings to increase the stray loss and overheat the adjacent structural members, the tank and the like.